1. Field of the Invention
This invention relates to a process for preparing industrially an .alpha., .beta.-unsaturated carboxylic: acid ester using at least one acid ester selected from an .alpha.-hydroxycarboxylic acid ester, an .alpha.-alkoxycarboxylic acid ester, and a .beta.-alkoxycarboxylic acid ester as starting materials.
2. Description of the Related Arts
The .alpha., .beta.-unsaturated carboxylic acid ester is very useful in industry as a starting material for synthetic resins. In particular, methyl methacrylate obtained from methyl .alpha.-hydroxyisobutyrate, methyl .alpha.-methoxyisobutyrate or methyl .beta.-methoxyisobutyrate have been important for practical use because methyl methacrylate can be used as a starting material for polymethyl methacrylate having excellent weather resistance and transparency.
As for a process of preparing .alpha., .beta.-unsaturated carboxylic acid or its ester, a process in which a dehydrating reaction of an .alpha.-hydroxycarboxylic acid ester is carried out in a liquid phase, has heretofore been disclosed in, for example, U.S. Pat. No. 3,487,101.
In addition, in Japanese Patent Publication No. 184047/1985, a process is disclosed in which methyl methacrylate is prepared by reacting concentrated sulfuric acid having a concentration of 90 to 100% with methyl .alpha.-hydroxyisobutyrate in a liquid phase.
However, in the process for preparing methacrylic acid ester using sulfuric acid, there are great difficulties in practical application at an industrial scale because an extremely excess amount of highly concentrated sulfuric acid is required and the problem of processing a large amount of waste sulfuric acid diluted with water formed by the reaction is involved.
On the other hand, a process has been proposed for preparing methyl methacrylate from methyl .alpha.-hydroxyisobutyrate by a vapor phase catalytic reaction using a solid catalyst such as phosphate.
For example, in Japanese Patent Publication No. 20611/1969, No. 20612/1969 and No. 15724/1970, there are disclosed processes in which high boiling point materials such as methyl .alpha.-hydroxyisobutyrate, methyl .alpha.-methoxyisobutyrate and methyl .beta.-methoxyisobutyrate among the impurities contained in crude methyl methacrylate synthesized by the acetone cyanohydrin method, are passed through a catalytic layer wherein solid phosphoric acid, alkali metal salts of phosphoric acid, or alkaline earth metal salts of phosphoric acid deposited on silica or silica-alumina to obtain methyl methacrylate and methacrylic acid.
However, when these phosphate catalysts are used, a high reaction temperature is required so that deposition of a large amount of carbonaceous material and a side reaction such as a hydrogenation reaction occur causing problems in that, for example, methyl isobutyrate is formed as a by-product in a large amount, and therefore the above described processes cannot be satisfactory for practical use.